He's big at Westminster (3)
I believe the answer is:
ben
'he's' is the definition.
(I've seen this in another clue)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ben that I've seen before include "& 3 Across Epic film" , "Tropical tree" , "High mountain in Scotland" , "inner room in Oban, say" , "Scottish 11" .)